Phonon Radiation Research Articles

Spin radiation of electrons, excitons, and phonons

Spin radiation of electrons, excitons, and phonons

Singular Points of the Radiation Spectrum of Leaky Surface Magnon Polarons

Within the dissipation-free approach, for a magnetic layer separating two semi-bounded ideal fluids, it is shown that, at the point of the phonon radiation spectrum where leaky surface magnon polarons of the “dark” state are formed (of interference or symmetry-protected type) at the interface with an acoustically less dense medium, both the numerator and the denominator of the input wave impedance can independently become zero (at the very point of existence of a bound state in the continuum, they vanish simultaneously). The calculation was performed for a two-sublattice model of an antiferromagnet, which simultaneously takes into account magnetoelastic, inhomogeneous exchange, and hyperfine coupling. Conditions have been found under which the mechanisms of formation of bound states in the spectrum of phonon radiation of leaky magnon polarons involving quasi-electronic or quasi-nuclear magnons are fundamentally different: elastodynamic or elastostatic.

Impact of material-dependent radiation – longitudinal optical phonon interaction on thermal electric-dipole radiation from surface metal − semiconductor grating structures

Infrared thermal radiation emission in the 8.5 – 28 THz frequency region is obtained using surface metal–semiconductor grating structures on undoped (u-) GaAs, u-GaP, u-ZnO, u-GaN, and n-type SiC in a temperature range of 430 – 630 K. These emissions resonate with longitudinal optical (LO) phonon or LO-like lattice vibration energies determined by the zero points of the real parts of the dielectric functions in the surface structures. The emissions of materials with Reststrahlen bandwidths of a few tens of cm−1 show the emissions resonating with their LO phonon modes, while materials with bandwidth of more than 170 cm−1 show peak energies significantly lower than the LO phonon: LO-like phonon resonance. The emission intensity is found to be dominated by the balance of radiative and nonradiative LO or LO-like phonon annihilation rates. The radiative rate is dominated by the LO-phonon–radiation interaction Hamiltonian and the Bose-Einstein factor. High emission intensity is obtained for the structure on u-ZnO with intense LO-like phonon–radiation interaction. The dependence of the emission intensity on temperature and emission window width for various materials shows the effect of material-dependent metal/semiconductor interface conditions on the emission efficiency.

Dissipative processes at the acoustic horizon

A transonic fluid flow generates an acoustic hole that is the hydrodynamic analogue of a gravitational black hole (BH). Acoustic holes emit a detectable thermal radiation of phonons at a characteristic Hawking temperature. The crucial concept is that the spontaneous phonon emission at the horizon produces an irreversible heat increase at the expenses of the bulk fluid kinetic energy. We show that such process can be described in terms of effective shear and bulk viscosities that are defined close to the horizon. We analyze this quantum friction process by resorting to a general kinetic theory approach as well as by the specific description of phonon emission as a tunneling process. The celebrated Kovtun, Son and Starinets (KSS) universal lower bound η/s=1/4π of the shear viscosity coefficient to entropy density ratio, readily follows, and is extended to the longitudinal bulk viscosity at the horizon. We come to the same saturation of the KSS bound after considering the shear viscosity arising from a perturbation of the background metric at the acoustic horizon providing a—in principle testable—realization of the so called BH membrane paradigm.

Akusticheskie solitony v gelikoidakh i spiral'nykh nanolentakh grafena

The dynamics of local regions of longitudinal compression in graphene helicoids and spiral carbon nanoribbons has been numerically simulated. It has been shown that a supersonic acoustic soliton can constantly move without radiation of phonons only in helicoids with transverse radius R 0.62 nm. Dimensionless velocity s of the soliton in this case falls into the interval 1.0–1.4. In larger radius helicoids and all spiral carbon nanoribbons, the motion of a soliton-like excitation is always accompanied by the intense radiation of phonons (the more the size of the spiral structure, the more intense the radiation).

Nonlinear relaxation between magnons and phonons in insulating ferromagnets in the external magnetic field

Nonlinear relaxation between spin waves (magnons) and the crystal lattice (phonons) in an insulating ferromagnet in the external magnetic field is investigated theoretically. Magnons and phonons are described by the equilibrium Bose–Einstein distributions with different temperatures Ts > Tl. The magnon temperature is assumed to be much lower than the Debye temperature Ts<<ΘD, which is justified at low temperatures. The nonlinear heat current from magnons to phonons is calculated microscopically in terms of the Cherenkov radiation of phonons by magnons. The results are discussed in comparison with the well-known theoretical results on the nonlinear electron-phonon relaxation in metals (M. I. Kaganov, I. M. Lifshitz, and L. V. Tanatarov [Zh. Eksp. Teor. Fiz. 31, 232 (1956)]). The elaborated theoretical description is relevant for spinpumping experiments and thermoelectric devices in which the magnon temperature is essentially higher than the phonon one. The derived expression for the heat current can be used for calculation of the nonlinear heat boundary resistance in spin-caloritronic heterostructures.

Exciton and Phonon Radiative Linewidths in Monolayer Boron Nitride

Exciton and Phonon Radiative Linewidths in Monolayer Boron Nitride

Noncontact friction: Role of phonon damping and its nonuniversality

While obtaining theoretical predictions for dissipation during sliding motion is a difficult task, one regime that allows for analytical results is the so-called noncontact regime, where a probe is weakly interacting with the surface over which it moves. Studying this regime for a model crystal, we extend previously obtained analytical results and confirm them quantitatively via particle based computer simulations. Accessing the subtle regime of weak coupling in simulations is possible via use of Green-Kubo relations. The analysis allows to extract and compare the two paradigmatic mechanisms that have been found to lead to dissipation: phonon radiation, prevailing even in a purely elastic solid, and phonon damping, e.g., caused by viscous motion of crystal atoms. While phonon radiation is dominant at large probe-surface distances, phonon damping dominates at small distances. Phonon radiation is furthermore a pairwise additive phenomenon so that the dissipation due to interaction with different parts (areas) of the surface adds up. This additive scaling results from a general one-to-one mapping between the mean probe-surface force and the friction due to phonon radiation, irrespective of the nature of the underlying pair interaction. In contrast, phonon damping is strongly nonadditive, and no such general relation exists. We show that for certain cases, the dissipation can even {\it decrease} with increasing surface area the probe interacts with. The above properties, which are rooted in the spatial correlations of surface fluctuations, are expected to have important consequences when interpreting experimental measurements, as well as scaling with system size.

Characterization of self-heating in cryogenic high electron mobility transistors using Schottky thermometry

Cryogenic low-noise amplifiers based on high electron mobility transistors (HEMTs) are widely used in applications such as radio astronomy, deep space communications, and quantum computing. Consequently, the physical mechanisms governing the microwave noise figure are of practical interest. In particular, the magnitude of the contribution of thermal noise from the gate at cryogenic temperatures remains unclear owing to a lack of experimental measurements of thermal resistance under these conditions. Here, we report measurements of gate junction temperature and thermal resistance in a HEMT at cryogenic and room temperatures using Schottky thermometry. At temperatures ∼20 K, we observe a nonlinear trend of thermal resistance vs power that is consistent with heat dissipation by phonon radiation. Based on this finding, we consider heat transport by phonon radiation at the low-noise bias and liquid helium temperatures and estimate that the thermal noise from the gate is several times larger than previously assumed owing to self-heating. We conclude that without improvements in thermal management, self-heating results in a practical lower limit for microwave noise figure of HEMTs at cryogenic temperatures.

Фононный спектр LED-гетероструктуры InGaN/GaN с квантовыми ямами

Measurements of the phonon spectrum of the LED heterostructure based on the In0.12Ga0.88N/GaN barrier showed the presence in it of four peaks of phonon radiation with energies of 0.193, 0.207, 0.353, and 0.356 eV. From a comparison of the results of the calculation of the energy spectrum of the electron and hole quantum wells with the obtained experimental data, it was assumed that these peaks can be interpreted as the energies of phonons generated during the capture of electrons from the barrier layer to the second level of dimensional quantization, as well as during the relaxation of electrons from the second level to the radiation level and trapping holes to the upper level of the quantum well.

Coupling Mesoscopic Boltzmann Transport Equation and Macroscopic Heat Diffusion Equation for Multiscale Phonon Heat Conduction

ABSTRACT Phonon heat conduction has to be described by the Boltzmann transport equation (BTE) when sizes or sources are comparable to or smaller than the phonon mean free paths (MFPs). When domains much larger than MFPs are to be treated or when regions with large and small MFPs coexist, the computation time associated with full BTE treatment becomes large, calling for a multiscale strategy to describe the total domain and decreasing the computation time. Here, we describe an iterative method to couple the BTE, under the Equation of Phonon Radiative Transfer approximation solved by means of the deterministic Discrete Ordinate Method, to a Finite-Element Modeling commercial solver of the heat equation. Small-size elements are embedded in domains where the BTE is solved, and the BTE domains are connected to a domain where large-size elements are located and where the heat equation is applied. It is found that an overlapping zone between the two types of domains is required for convergence, and the accuracy is analyzed as a function of the size of the BTE domain. Conditions for fast convergence are discussed, leading to the computation time being divided by more than five on a study case in 2D Cartesian geometry. The simple method could be generalized to other types of solvers of the Boltzmann and heat equations.

Three-Dimensional Ballistic-Diffusive Heat Transport in Silicon: Transient Response and Thermal Conductivity

Abstract Phonons are the main contributors to thermal energy transfer in thin films. The size dependence of the thermal transport characteristics alters the film properties such as thermal conductivity. Hence, in the present study, three-dimensional, transient phonon transport in dielectric material is studied through the Equation of Phonon Radiative Transport (EPRT) to assess the size dependence of thermal conductivity. The numerical scheme is introduced solving the EPRT in three dimensions and the governing algorithm is described in detail. A parametric study is carried out examining the effect of the Kn \mathrm{Kn} number on the thermal energy transport characteristics in three-dimensional thermally excited film. The formulation and estimation of the effective thermal conductivity tensor is presented and discussed, thereby extending, to some extent, the one-dimensional results obtained earlier. We demonstrate that thermal conductivity changes in all directions, depending on the size effect. In addition, the directions of the temperature gradient and heat flux vectors differ as the Kn \mathrm{Kn} number approaches unity.

Thermal conductivity reduction by scallop shaped surface modulation in silicon nanowires

We report the reduction of thermal conductivity and phonon mean-free-path below the Casimir limit originated by phonon backscattering in silicon nanowires with scallop shaped surface modulation. The scallop surface modulation with a period of 55–110 nm on 200 nm diameter nanowires was created using top-down deep-reactive-ion-etching. The measured thermal conductivity was 15.13 W m−1 K−1, 34% lower than those of nanowires fabricated by metal-assisted-chemical-etching with a similar diameter, and 60% lower than the Casimir limit. An analysis using the phonon Boltzmann transport equation and radiation analogy of ballistic phonons shows that this drastic reduction stems from the phonon backscattering at the scallop surface. The thermal conductivity measured at 300–500 K showed a nearly temperature-independent trend suggesting that the structural control can overcome a material limit. Our results provide a systematic nanostructure control approach to improve the thermoelectric efficiency more than the limit imposed by a material.

Thermal Characterization and Modelling of AlGaN-GaN Multilayer Structures for HEMT Applications

To optimize the thermal design of AlGaN-GaN high-electron-mobility transistors (HEMTs), which incorporate high power densities, an accurate prediction of the underlying thermal transport mechanisms is crucial. Here, a HEMT-structure (Al0.17Ga0.83N, GaN, Al0.32Ga0.68N and AlN on a Si substrate) was investigated using a time-domain thermoreflectance (TDTR) setup. The different scattering contributions were investigated in the framework of phonon transport models (Callaway, Holland and Born-von-Karman). The thermal conductivities of all layers were found to decrease with a temperature between 300 K and 773 K, due to Umklapp scattering. The measurement showed that the AlN and GaN thermal conductivities were a magnitude higher than the thermal conductivity of Al0.32Ga0.68N and Al0.17Ga0.83N due to defect scattering. The layer thicknesses of the HEMT structure are in the length scale of the phonon mean free path, causing a reduction of their intrinsic thermal conductivity. The size-effect of the cross-plane thermal conductivity was investigated, which showed that the phonon transport model is a critical factor. At 300 K, we obtained a thermal conductivity of (130 ± 38) Wm−1K−1 for the (167 ± 7) nm thick AlN, (220 ± 38) Wm−1K−1 for the (1065 ± 7) nm thick GaN, (11.2 ± 0.7) Wm−1K−1 for the (423 ± 5) nm thick Al0.32Ga0.68N, and (9.7 ± 0.6) Wm−1K−1 for the (65 ± 5) nm thick Al0.17Ga0.83N. Respectively, these conductivity values were found to be 24%, 90%, 28% and 16% of the bulk values, using the Born-von-Karman model together with the Hua–Minnich suppression function approach. The thermal interface conductance as extracted from the TDTR measurements was compared to results given by the diffuse mismatch model and the phonon radiation limit, suggesting contributions from inelastic phonon-scattering processes at the interface. The knowledge of the individual thermal transport mechanisms is essential for understanding the thermal characteristics of the HEMT, and it is useful for improving the thermal management of HEMTs and their reliability.

Coherent Manipulation of Single Electrons with Optical Photons in Condensed Helium‐4

AbstractSingle electrons are one of the simplest quantum objects in nature. In free space or conventional materials, single electrons hardly interact with optical (ultraviolet‐visible‐infrared) photons, so their quantum states cannot be easily manipulated by optics. Here, a notable exception is theoretically demonstrated: a single electron in condensed helium‐4 can be self‐confined in a nanometric bubble of 1 to 2 nm radius and resonantly interact with mid‐infrared photons of 3 to 12 µm wavelength. A femtosecond laser can drive the electron into Rabi oscillations and prepare it onto arbitrary superposition states between the ground and excited states. After the laser pulse, the electron bubble can exhibit remarkable nonequilibrium quantum dynamics and evolve toward various final states with various lifetimes. These processes are accompanied with intense phonon radiation.

Čerenkov phonon radiation and phonon structure in electron emission, electroluminescence, and current–voltage characteristics of electroformed Al–Al2O3–Ag diodes

Electroforming of metal–insulator–metal (MIM) diodes is a non-destructive dielectric breakdown that results in a conducting filament in the insulator and changes the high resistance of the as-prepared MIM diode into a low-resistance state. It is a critical step in the development of conducting states between which switching can occur in memristors or resistive switching memories. Electroforming of Al–Al2O3–Ag diodes in vacuum results in the formation of a conducting Al2O3 filament. There are Ohmic contacts at the Al–Al2O3 and Al2O3–Ag interfaces. Electronic processes develop, such as voltage-controlled negative resistance in the current–voltage (I–V) characteristics, electron emission into vacuum (EM), and electroluminescence (EL). I–V curves, EM, and EL of three electroformed Al–Al2O3–Ag diodes with anodic Al2O3 thicknesses between 36 nm and 49 nm have been measured with 20 mV resolution between voltage steps. Periodic voltage structure occurs for the three electronic processes for applied voltages between the voltage threshold for EL, ∼2 V, and the maximum applied voltage, 11 V. The voltage peak period, 60 mV to 70 mV, is the same as the range of LO phonon energies of anodic Al2O3. The proposed mechanism is Čerenkov phonon generation by ballistic electrons whose velocity exceeds the velocity of sound in Al2O3. The phonons, in turn, modify the motion and number of electrons that are emitted into vacuum, that are responsible for EL radiation, and that contribute to conduction through the diode. The occurrence of LO phonons shows that the conducting filament is Al2O3, not Ag or Al. The phenomena may provide a new method of generating terahertz radiation.

Dynamical friction in superfluids

We compute the dynamical friction on a small perturber moving through an inviscid fluid, i.e., a superfluid. Crucially, we account for the tachyonic gravitational mass for sound waves, reminiscent of the Jeans instability of the fluid, which results in non-zero dynamical friction even for subsonic velocities. Moreover, we illustrate that the standard leading order effective theory in the derivative expansion is in general inadequate for analysing supersonic processes. We show this in two ways: (i) with a fluid treatment, where we solve the linearized hydrodynamical equations coupled to Newtonian gravity; and (ii) with a quasiparticle description, where we study the energy dissipation of a moving perturber due to phonon radiation. Ordinarily a subsonic perturber moving through a superfluid is kinematically prohibited from losing energy, however the Jeans instability modifies the dispersion relation of the fluid which can result in a small but non-vanishing dynamical friction force. We also analyzed the soft phonon bremsstrahlung by a subsonic perturber scattered off an external field.

ENTROPY GENERATION RATE IN A MICROSCALE THIN FILM

This paper presents a new formulation of the rate of entropy generation in thin films whose thickness is of the order of the mean-free-path or less. In this relation, an expression for the gradient of the equivalent equilibrium temperature is proposed that is a function of the gradient of the phonon intensity at any point inside the thin film. It is shown that the proposed expression reduces to the familiar gradient of the thermodynamic temperature in the diffusive limit. Furthermore, the new formulation is used to compute the entropy generation rate for the case of steady-state, one-dimensional heat transfer in a thin film by first solving the Equation of Phonon Radiative Transfer to determine the phonon intensity. These computations are performed both for the silicon and the diamond thin films, for a range of Knudsen numbers starting from the diffusive limit up until the ballistic limit. It is found that the entropy generation rate attains a peak value at Kn = 0.7 and decreases for other Knudsen numbers when non-equilibrium transport is adopted in the analysis. However, rate of entropy generation increases almost linearly for the equilibrium heating situation. This is true for both the silicon and the diamond thin films.

Achieving Huge Thermal Conductance of Metallic Nitride on Graphene Through Enhanced Elastic and Inelastic Phonon Transmission.

Low thermal conductance of metal contacts is one of the main challenges in the thermal management of nanoscale devices of graphene and other two-dimensional (2D) materials. Previous attempts to search for metal contacts with high thermal conductance yielded limited success because of the incomplete understanding of the origins of low thermal conductance. In this paper, we carefully study the intrinsic thermal conductance of metal/graphene/metal interfaces to identify the heat transport mechanisms across graphene interfaces. We find that unlike metal/diamond interfaces, the intrinsic thermal conductance of most graphene interfaces (except Ti and TiN x) is only ≈50% of the phonon radiation limit, suggesting that heat is carried across graphene interfaces mainly through the elastic transmission of phonons. We thus propose a convenient approach to substantially enhance the phononic heat transport across metal contacts on graphene, by better matching the energy of phonons in metals and graphene, for example, using metallic nitrides. We test the idea with TiN xwith phonon frequencies of up to 1.2 × 1014 rad/s, 39% of the highest phonon frequencies in graphene of 3.1× 1014 rad/s. Interestingly, we obtain a huge thermal conductance of 270 MW m-2 K-1 for the TiN x/graphene interface, which is ≈140% of the phonon radiation limit. Thus, the huge thermal conductance cannot be fully explained by enhanced elastic phonon transport alone, but may be partially attributed to inelastic phonon transport across the TiN x/graphene interface. Our work provides guidance for the search for good metal contacts on 2D materials and devices.

Nonlinear relaxation between magnons and phonons in insulating ferromagnets

Nonlinear relaxation between spin waves (magnons) and the crystal lattice (phonons) in an insulating ferromagnet is investigated theoretically. Magnons and phonons are described by the equilibrium Bose-Einstein distributions with different temperatures. The nonlinear heat current from magnons to phonons is calculated microscopically in terms of the Cherenkov radiation of phonons by magnons. The results are discussed in comparison with the well-known theoretical results on the nonlinear electron-phonon relaxation in metals [Kaganov, Lifshitz, Tanatarov, J. Exp. Theor. Phys. 31, 232 (1956)]. The elaborated theoretical description is relevant for spin-pumping experiments and thermoelectric devices in which the magnon temperature is essentially higher than the phonon one.